Oxygen gas sensors are usable in automobile exhaust systems to monitor the oxygen content and thus the efficiency of the engine operation. A type of sensor is one, in the form of a thimble, made from a solid electrolyte which may be formed of zirconium dioxide which has conductive layers, such as a layer of platinum, on the inner surface and the outer surface. While the conductive layer on the inner surface is normally subjected to atmospheric conditions and relatively stable, the conductive layer on the outer surface is subject to the degradative effects of the high temperature flow of the exhaust gas which can lead to gradual loss of the conductive layer and decrease in the performance of the sensor. Loss of the conductive layer, such as platinum, is a particularly severe problem in automotive exhaust measurements where the large temperature and gas composition fluctuations speed the degradation. It has been proposed to place a porous protective coating over the conductive layer in order to protect that layer, such as an oxide film of magnesium-aluminum spinel, but such measures, while retarding loss of the platinum, still do not extend the life of the sensor sufficiently. Formation of a porous protective layer over the conductive layer is described in U.S. Pat. Nos. 3,645,875 and 3,978,006. Such an overlain protective layer, however, does not eliminate one of the primary reasons for platinum loss, the reaction of platinum and the solid oxide electrolyte. Under chemically reducing conditions, for example, which are found in automobile exhausts, the platinum and zirconium dioxide, when used as the electrolyte, react to form intermetallic platinum-zirconium compounds at their interface. Under subsequent oxidizing conditions, found in the same exhaust system, these compounds can revert to platinum and zirconium dioxide with a rupturing of the bonds that initially held the platinum onto the zirconium dioxide.
Automobile exhaust gases are known to vary from chemically reducing to oxidizing conditions during engine operation depending upon the driving conditions. Under accelerating conditions, relatively large amounts of unburnt hydrocarbons, carbon monoxide and hydrogen are present in the exhaust and even elemental carbon may deposit on or near the sensor. All of these components are chemically reducing and can lead to the intermetallic formation between platinum and zirconium dioxide (or other oxide electrolytes).
The problem of loss of the conductive layer, such as platinum, on a solid electrolyte is discussed in U.S. Pat. No. 3,989,614 which teaches use of a porous electronic conductor over the outer surface of a solid electrolyte tube, where the electronic conductor is a transition metal oxide. The porous coating is of an electronic conductor impervious to oxygen ions and hydrogen molecules. This porous coating is applied to the surface of the solid electrolyte in such a manner that voids are present in the coating, with solid electrolyte being exposed which is then directly contacted with the platinum catalyst that is subsequently placed over the sensor. There are thus areas on the surface of the electrolyte which are coated by the transition metal oxide and the catalyst, areas which are coated by the transition metal oxide only, and other areas which are covered by the platinum catalyst only, as clearly shown in the drawing of that patent. While this system employs a coating of transition metal oxide over the solid electrolyte and a catalyst layer therefor, as described, there are specific areas of direct contact of platinum catalyst and solid electrolyte at which intermetallic platinum-electrolyte compound formation and subsequent degradation could occur.
An object of the present invention is to provide an improved solid electrolyte sensor where reaction between the solid electrolyte and the catalyst layer is prevented by the use of a porous, nonreactive but compatible protective bonding layer between those components.
The introduction of a protective bonding layer will also improve the catalytic nature of the platinum electrode. Oxygen gas sensors in automobile exhausts ordinarily are designed to measure the net, equilibrium amount of oxygen in the hot gas. Thus, the outer surface of the sensor must be catalytic to complete all possible combustion reactions in the gas to be measured. The outer platinum electrode commonly acts as a catalyst as well as an electrical conductor. The catalytic efficiency of the platinum electrode will greatly depend upon its surface area. While such electrodes are initially applied in a manner that will give it the greatest surface area possible on the limited area of the sensor element, with continued used at high temperatures, the platinum slowly agglomerates so as to reduce the exposed surface area thereof. This is a further form of degradation of the sensor. With the use of the porous, nonreactive but compatible protective bonding layer of the present invention, the surface area of the platinum electrode is greatly increased and the efficiency of its catalytic activity prolonged.
With the use of the protective bonding layer interposed between the solid electrolyte and the platinum catalyst, the availability of embedding particles of platinum in the surface of the protective bonding layer further improves adherence of the platinum electrode to the sensor.